This section introduces aspects that may help facilitate a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is, or what is not, prior art.
Systems employing integrated photonic circuits can provide powerful platforms for ultra-wide-band signal processing. Silicon-based integrated photonic circuits hold a particularly promising future for high-level integration of photonic circuits. For example, integrated photonic circuits are used in both the transmitters and receivers of modern optical transmission systems.
Polarization diversity is an important requirement of many optical circuits; polarization diverse circuits process light independent of its polarization. Such circuits are capable of supporting modern plural polarization modulation techniques such as the recently developed dual-polarization quadrature phase-shift keying (DPQPSK) advanced modulation technique. Plural polarization modulation techniques allow optical fibers and monolithic waveguides to carry more data than they could previously, resulting in very high data transmission rates.
A polarization beam splitter (PBS) is useful for implementing polarization diversity. A PBS splits light traveling in a waveguide into transverse electric (TE) and transverse magnetic (TM) polarization components. The TE and TM polarization components can thereafter be processed in separate ways that take into account their differing physical characteristics.
A PBS is often constructed from a single imbalanced Mach-Zehnder interferometer. However, an imbalanced Mach-Zehnder interferometer does not allow each polarization component to be adjusted independently. This substantially limits its utility in modern optical transmission systems.
A PBS may also be constructed from a single evanescent coupler formed in silicon and having large differences TE and TM group indices of refraction. The advantage of an evanescent coupler is that it is compact and its operation is relatively simple. Unfortunately, it is relatively difficult and expensive to manufacture and is typically wavelength-dependent, which narrows its effective bandwidth and therefore its utility in modern optical transmission systems.